Method and apparatus for measuring optical retardation in transparent materials

ABSTRACT

A method and apparatus for measuring optical retardation of a transparent material, such as the protective transparent cover sheet or substrate which protects the recording layer of an optical or magneto-optical disk. The method of the invention involves the steps of producing a beam of plane-polarized radiation in which the plane of polarization rotates continuously, directing such beam through a sample of transparent material where optical retardation is to be measured, and continuously comparing the angle of polarization of the beam entering the sample with the angle of plane polarization of the beam exiting the sample. Preferably, the beam and sample are continuously moved relative to another, whereby the optical retardation is measured at different points on the sample. The beam-producing step may be effected by passing a beam of circularly polarized radiation through a plane-polarizing filter while continuously rotating the filter. In this case, the comparing step may be achieved by (a) redirecting the beam exiting the sample back through the rotating filter and (b) monitoring the intensity of the beam after passing back through such filter.

BACKGROUND OF THE INVENTION

This invention relates to method and apparatus for detecting andmeasuring optical retardation in transparent materials. The method andapparatus of the invention are particularly well suited for measuringoptical retardation variations in a transparent sheet whose thicknessand/or birefringence may vary from point-to-point over the area of thesheet.

For a variety of reasons, it is often desirable to measure the opticalretardation in a sheet of transparent material. For example, in thefield of optical recording, the recording layer of an optical disk isusually protected from dust and dirt by a transparent cover sheet orsubstrate. To recover the information recorded on the optical disk, aplane-polarized beam, as produced by a laser, is directed at therecording layer through the protective, transparent cover sheet orsubstrate. Upon being reflected by the recording layer (or a reflectivelayer underlying the recording layer) the laser beam is directed to aphotodetector which senses the data-produced intensity variations of thebeam. To isolate the laser cavity from radiation reflected from the diskduring readout, it is common to employ the combination of a polarizingbeam-splitter and a quarter-wave plate. Radiation from the read laser ispolarized in a given plane which allows it to pass through thepolarizing beam-splitter. Such plane-polarized beam is then circularlypolarized in a, say, clockwise sense by passing it through thequarter-wave plate. Upon being reflected from the disk, the beam becomescircularly polarized in the opposite sense and, upon passing through thequarter-wave plate a second time, becomes plane polarized at an angleperpendicular to the plane of polarization passed by the polarizingbeam-splitter. Upon striking the beam-splitter the second time with itsplane of polarization perpendicular to that passed by the beam splitter,100% of the beam is reflected to the photodetector. Obviously, if thestate of polarization of the radiation returning to the beam-splitter isanything other than plane-polarized in a direction perpendicular to thatpassed by the beam-splitter, the beam-splitter will pass a portion ofsuch radiation back to the laser cavity, causing undesired variations inthe laser output and, moreover, effecting a reduction and modulation ofthe radiation striking the data and servo detectors. Any opticalretardation of the beam between the two passes through the quarter-waveplate will cause some degree of ellipticity in the polarization of thebeam, and some energy will return to the laser cavity. A major source ofsuch retardation is birefringence in the protective transparent layer ofthe optical disk. Such birefringence can be produced during themanufacture of the transparent layer or can be produced by non-uniformstressing of the layer during assembly of the disk. Before an opticaldisk is approved for shipping, core must be taken that the opticalretardation introduced by the transparent protective layers meetscertain strict standards. Preferably, the optical retardation introducedby the transparent layers should be no greater than 0.1 λ (i.e., thewavelength of the readout laser (830 nm.)).

A known method for measuring the amount of retardation in an opticalelement is a point-by-point technique which makes use of a device knownas a polariscope. According to this method, the beam is passed throughboth the sample and the polariscope, and the polariscope is adjusted toadd an equal and opposite amount of retardation so that the net effectis zero retardation. To do this, it is necessary to precisely align theoptical axes of the sample and the polariscope 90° apart. This presentsno problem when measuring the retardation of a single point on thesample; however, when it is desired to make measurements at, say, a 1000points over the surface of the sample, this technique is extremelytime-consuming and operator-dependent for accuracy.

SUMMARY OF THE INVENTION

In view of the foregoing, an object of this invention is to provide amethod and apparatus for making a rapid and highly reliable measurementsof the optical retardation at a multiplicity of points on a transparentelement.

The method of the invention involves the steps of producing a beam ofplane-polarized radiation in which the plane of polarization rotatescontinuously, directing such beam through the transparent element, andcomparing the angle of plane polarization of the beam entering theelement with the angle of plane polarization of the beam exiting suchelement. The apparatus of the invention comprises means for producing abeam of plane polarized radiation in which the plane of polarizationrotates continuously, means for directing such beam through atransparent layer whose optical retardation is to be measured, and meansfor comparing the angle of plane polarization of the beam entering thelayer with the angle of plane polarization of the beam exiting thelayer.

The invention will be better understood from the ensuing detaileddescription of preferred embodiments, reference being made to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are cross-sectional illustrations of conventional opticaldisk assemblies; and

FIG. 3 is a schematic illustration of preferred apparatus forimplementing the method of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to the drawings, FIGS. 1 and 2 are fragmentary cross-sectionalillustrations of conventional optical disks. In the FIG. 1 illustration,the optical disk is shown to comprise a rigid opaque substrate OS havinga reflective layer R thereon. A recording layer RL overlies thereflective layer and comprises, for example, a die/binder material. Whensubjected to an intensity-modulated laser beam of suitable intensity,pits P representing recording information, are ablated in recordinglayer. These pits are optically detected by scanning the recording layerwith a non-modulated beam B of radiation and detecting the radiationreflected from the reflective surface which underlies the recordinglayer. To protect the recording layer from dust and dirt as well as todisplace such dust and dirt particles out of the focal plane of theread/write laser beam, a transparent cover sheet CS may be provided.Such cover sheet may be physically separated from the recording layer byan airspace A. Alternatively, the protective cover sheet may comprise arelatively thick transparent layer formed directly on the recordinglayer.

In FIG. 2, another form of optical disk is shown to comprise a pair ofconfronting transparent substrates TS separated by airspace A. Eachsubstrate TS is provided, as above, with a recording layer RL which, inthis case, is overcoated with a reflective layer R. Here again,recording and playback are effected through a transparent element, inthis case, the transparent substrate TS.

In optical disk structures of the type described above, it is essentialthat the transparent elements CS and TS exhibit relatively low opticalretardation, preferably, less than one-tenth of the wavelength of thelaser beam used to readout the recorded information; otherwise, as notedabove, the plane polarization of the read beam will be altered to theextent that a portion of the beam will return to the laser cavityundesired intensity variations of the laser beam, as well as undesiredvariations in the level of radiation reaching the data and servodetectors.

According to present invention, there is provided a method and apparatusfor rapidly and reliably measuring the optical retardation oftransparent elements, such as the transparent cover sheet and substratesin the above optical disk assemblies. The method lends itself toproducing continuous curves of double pass retardation measurements at amultitude of points along a path, for example, the circular path definedby a constant radius about a point. Importantly, the method of theinvention is inherently insensitive to the orientation of the major andminor axes of the sample whose retardation is being measured. Inaddition, it is self-calibrating.

Referring to FIG. 3 which schematically illustrates preferred apparatusfor implementing the method of invention, a beam B of monochromaticradiation emanating from a laser L, such as a helium-neon laser, isfirst passed through a plane-polarizing element PP1. Element PP1 simplyassures the plane-polarization of beam B. The beam exiting element PP1is then passed through a quarter-wave plate QWP to produce a beam B' ofcircularly polarized radiation in which the intensity of the beam isequal at all angles of polarization. Beam B' then passes through asecond plane-polarizing element PP2 which is rotating at a moderate rate(e.g. 1000 rpm) about an axis normal to its surface. Plane-polarizingelement PP2 is rigidly coupled to one end of a hollow shaft HS which isrotatably driven by a motor M. Beam B' is directed through the hollowshaft to element PP1. As the plane-polarizing element PP2 rotates at aconstant rate, the angle of polarization of the beam B" exiting elementPP2 rotates at a corresponding rate. Beam B" is then directed throughthe transparent sample whose retardation is to be measured, whereupon itstrikes a reflector R positioned at a slight angle relative to the pathof travel of the incident beam. Upon reflecting from the reflectivesurface R, beam B" passes again through the sample S and back throughthe plane polarizing element PP2 to a photodetector D.

In using the above apparatus, it will be appreciated that, in the eventthat portion of the sample through which beam B" passes produces nooptical retardation, the plane of polarization of beam B" will be thesame both before and after it interacts with the sample. Since thepolarizer PP2 is rotating slowly, relative to the travel time ofradiation to and from the reflector R, the beam will pass back throughthe polarizer unaffected. Of course, some attenuation occurs due to thedensity characteristics of polarizers, but this change in intensity is aconstant. If, on the other hand, the sample causes a certain amount ofoptical retardation, for example, one-half wavelength after passingthrough the sample twice, then the output of detector D will bemodulated in intensity, such modulation being at a frequency four timesthat of the rotation rate of element PP2. When the plane of polarizationof beam B" as it strikes the sample is parallel with either of theoptical axis (major or minor) of the sample, no retardation will occurand the return beam will be unimpeded by polarizer PP2, as in the casewhere the sample introduces no retardation. However, when the plane ofpolarization of beam B" as it strikes the sample is at 45° to thesample's axes, the plane of polarization of the beam emerging from thesample will be rotated 90° relative to the incident beam due to thesample's retardation. In this case, the beam is completely blocked bythe rotating polarizer PP2. As will be appreciated, during each rotationof the polarizer PP2, there will be a 100% modulation of the beamintensity, such modulation occurring, as noted above, at four times therevolution rate of the polarizer. If the sample has a 1/4 waveretardation, the beam sensed by detector D will be modulated by 50%.Obviously, the amount of modulation of the beam intensity sensed by thephoto detector is proportional to the retardation introduced by thesample. The relationship between modulation, M, and retardation, R, isgiven by the relationship:

    Modulation=[cos (πR/λ)].sup.2

The output of detector D may be sampled rapidly (e.g. 1000 samples/sec.)by a computer-controlled voltmeter. The amount of modulation is thencalculated by the computer and converted to retardation. If, during themeasurement, the sample is moved, such as rotated about axes a, to allowthe beam to scan the sample, then a continuous trace of retardationalong the scan path is produced.

When using the above scheme to measure the optical retardation presentin the transparent cover sheet or substrate of an optical disk assembly,the optical disk assembly is positioned in the path of beam B". The diskis then rotated, for example, at 0.75 rpm, so that the opticalretardation can be measured at any desired radius on the disk. In thismanner, a continuous trace of the optical retardation at a given radiusis provided and, based on the depth of modulation of the signal, it maybe determined whether the transparent cover sheet or substrate meetscertain minimum standards.

The method for measuring optical retardation described above has provento be highly advantageous over the traditional point-by-point methods.In use, it has been found to be highly reliable, accurate, fast,operator-independent, and easily automated.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

I claim:
 1. A method for measuring optical retardation of a transparentelement, said method comprising the steps of:(a) producing a beam ofplane-polarized radiation in which the plane of polarization rotatescontinuously; (b) directing such beam through a transparent element; and(c) continuously comparing the angle of plane polarization of the beamentering the element with the angle of plane polarization of the beamexiting the element.
 2. The method as defined by claim 1 wherein saidbeam-producing step comprises the step of passing a beam of circularlypolarized radiation through a plane-polarizing filter while rotating theplane of polarization of such filter.
 3. The method as defined by claim2 wherein said comparing step comprises the steps of (a) redirecting thebeam exiting the element back through said filter and (b) monitoring theintensity of the beam after passing back through said filter.
 4. Themethod as defined by claim 3 wherein said re-directing step comprisesthe step of reflecting the beam exiting the element back through suchelement.
 5. Apparatus for measuring optical retardation of transparentmaterials, said apparatus comprising:(a) means for producing aplane-polarized beam of radiation in which the plane of polarizationcontinuously rotates. (b) means for directing such beam through atransparent sample whose optical retardation is to be measured; and (c)means for continuously comparing the angle of plane polarization of thebeam entering the sample with the angle of plane-polarization of thebeam exiting the sample.
 6. The apparatus as defined by claim 5 whereinsaid producing means comprises (a) means for producing a beam ofcircularly polarized radiation, (b) polarizing filter means positionedin said beam of circularly polarized radiation for converting said beamof circularly polarized radiation to a beam of plane-polarized radiationand (c) means for rotating said polarizing filter means.
 7. Theapparatus as defined by claim 6 wherein said comparing means comprisesmeans for directing the beam exiting the sample back through saidpolarizing filter means, and means for monitoring the intensity of thebeam after passing back through said filter means.
 8. Apparatus formeasuring optical retardation in a birefringent membrane overlying areflective surface, said apparatus comprising:(a) means for producing abeam of circularly polarized radiation; (b) polarizing filter meanslocated in said beam for converting said circularly polarized radiationto a first beam of plane-polarized radiation; (c) means for rotatingsaid polarizing filter means to cause the plane of polarization of saidfirst beam to rotate continuously; (d) means for scanning a birefringentmembrane overlying a reflective surface with said first beam to causesaid first beam to pass through said membrane, reflect from said surfaceand pass again through said membrane, whereby a second beam ofplane-polarized radiation is produced, the plane of polarization of saidsecond beam being angularly displaced with respect to the plane ofpolarization of said first beam proportional to the amount of opticalretardation; and (e) means for continuously monitoring the angle betweenthe respective planes of polarization of said first and second beams. 9.The apparatus as defined by claim 8 wherein said monitoring meanscomprises means for passing said second beam back through saidpolarizing filter means, and means for detecting the intensity of saidsecond beam after being passed by said polarizing filter means.